Fluid film bearings

ABSTRACT

PCT No. PCT/GB95/00955 Sec. 371 Date Oct. 25, 1996 Sec. 102(e) Date Oct. 25, 1996 PCT Filed Apr. 26, 1995 PCT Pub. No. WO95/29346 PCT Pub. Date Jan. 21, 1995A hydrodynamic fluid film bearing in which a plurality of circumferentially spaced bearing elements are provided, defining areas of support for the rotating part of the bearing. At least some of the bearing elements are adjustable during operation to vary lubrication conditions in the fluid film. The bearing elements are of sufficiently high stiffness that the position of the entire bearing surface of each element is essentially independent of the pressure in the fluid film encountered during operation. The bearing allows complete control of the lubrication conditions and can be adjusted for optimum performance over a wide range of speed and load.

FIELD OF THE INVENTION

The invention relates to fluid film bearings and, more specifically, tohydrodynamic bearings.

BACKGROUND OF THE INVENTION

In the most basic form of hydrodynamic bearing, a journal is rotatablymounted relative to a housing with a small radial clearancetherebetween. The axes of the journal and housing are offset by a smalldistance, such that in operation, when a lubricant at relatively lowpressure is introduced via a suitable opening in the housing, thejournal exerts a frictional drag on the lubricant drawing it into thewedge-shaped part of the space between the surfaces, and a convergentfilm is formed. The action of the relatively moving surfaces creates inthe convergent film a relatively high pressure zone which holds thesurfaces apart.

Establishing a lubricant film of satisfactory thickness is dependentupon a number of different conditions. During operation of such abearing, factors such as the operating speed and the operating loadaffect the lubrication conditions and hence the bearing performance.Over the life of the bearing, wear can clearly also affect itsperformance.

A more recent approach to hydrodynamic bearings has involved theprovision of segmental bearings, wherein the bearing surface is formedof a plurality of circumferentially spaced pads. Each pad is supportedin the housing for limited pivotal movement and during operation the padtilts such that the moments acting on it due to the fluid film pressuresare in equilibrium.

Examples of this type of bearing construction, known as a `tilting pad`bearing, are disclosed in U.S. Pat. No 4,714,357, U.S. Pat. No 4,490,054and U.S. Pat. No 4,636,095.

Although tilting pad bearings have application in a wide variety ofdifferent fields, they may exhibit disadvantages particularly at highrotational speeds and when subjected to transverse loads. At high speed,and especially under no-load or low-load conditions, oscillations of therotating shaft can be significant. Moreover, under transverse loads,there can be considerable changes in the eccentricity of the shaft (thedisplacement of the shaft axis from that of the housing). Both theseeffects are the result of the fact that the segmental bearingaccommodates oscillations and transverse movement by the pivoting of thetilting pads, and there is no possibility of intervention to control theoperation of the bearing apart from the provision of shims to alter theunloaded setting (sometimes known as the `preload`) of the bearing pads,which of course involves decommissioning the bearing to strip it down.Moreover, the load capacity at low speeds, such as at starting-up andslowing-down speeds, is very low, since the hydrodynamic lubricantpressure is low in the fluid film.

A further fluid film bearing arrangement is disclosed in GB-1 010 547,in which a circumferentially-spaced series of bearing elementssurrounding a shaft are mounted for limited radial displacement againsta linked series of pressure chambers. It is intended that any relativedisplacements as the shaft vibrates are effectively damped by the factthat each element can find its own equilibrium position, as determinedby the balance between the force produced by its associated pressurechamber and the opposing force resulting from the fluid film pressure.This arrangement provides no means of actually controlling theconditions in the fluid film. It can be said that the instantaneousfluid film pressure acts to determine the instantaneous position of thebearing elements.

Another bearing, and the method of its manufacture, is disclosed in GB-1251 160, in which a plurality of bearing surfaces support a rotatingshaft. The bearing surfaces can be radially moved while assembling thebearing, and then fixed in a support ring to set their bearing position.No further adjustment of any type is allowed or envisaged with thisbearing.

An alternative form of hydrodynamic sliding bearing and one appropriatespecifically to lubrication by a gas, such as air, is known as the foilbearing. In a foil bearing a plurality of flexible bearing foils aremounted pre-loaded against the shaft so as to wrap the shaft and hencecreate the convergent zones to provide a high pressure supporting gasfilm which separates the surface of the shaft from the surfaces providedby the foils.

Such a bearing is described in U.S. Pat. No 4,445,792, in which thecommon preload of all the foils may be adjusted, either as a singleaction or on a continuous basis, to respond to bearing parameters andalter the bearing stiffness, automatically or otherwise. Although thebearing described in this U.S Patent Specification provides usefuladjustment, even during operation, the control afforded over the preciselubrication conditions within the fluid film is very limited, as theflexible foils are mechanically conformal, that is, even if the preloadis altered, the low bending stiffness of a foil is such that the form ittakes is determined by the balance of forces on the foil. The foil shapetherefore depends not just on the preload and the foil initial geometryand material properties, but on the fluid film pressure distribution.The changes in the fluid film thickness due to the radial flexibility ofthe foils in such a bearing are of similar magnitude or even greaterthan the thickness of the fluid film itself.

In U.S. Pat. No 4,815,864 a tension foil bearing is described, in whichthe tension of each of three foils is independently adjustable to alterthe shape of the fluid film between the surfaces. In this way theselective adjustment can be used to maintain or shift the centre ofrotation of the shaft as desired. Once again, the foils are conformal.The shape of the fluid film is determined by, amongst other factors,flexing of the foil, and this cannot be controlled by selection but isdependent on conditions within the bearing itself, particularly on theoil film pressure. This is, in common with the bearing of U.S. Pat. No4,445,792, due to the fact that the adjustment is by means of adjustmentof a force acting on the fluid film in operation (i.e. the preload orfoil tension).

A further design of adjustable segmental bearing is disclosed inSU-A-174 042, which describes a plain bearing having segmental bearingsurfaces formed by tabs bendable by means of adjustment and fasteningscrews located in an outer race. The bearing is a rubbing bearing, theadjustment facility providing a means of compensating for wear.

SUMMARY OF THE INVENTION

In many applications it has been found that existing hydrodynamicbearings do not provide sufficient control on bearing performance over asufficiently wide range of speed and load conditions, and it is anobject of the present invention to provide a bearing in which thelubrication conditions can be controlled in a more satisfactory manner.

According to the invention, there is provided a hydrodynamic bearinghaving means for supporting a rotating part of the bearing in operationon a film of lubrication fluid, said means comprising a plurality ofcircumferentially spaced bearing elements having respective bearingsurfaces defining areas of support for the rotating part, at least someof the bearing elements being adjustable during operation to varylubrication conditions in the fluid film, wherein the adjustable bearingelements are of sufficiently high stiffness that the position of theentire bearing surface of a bearing element is essentially independentof the pressure in said fluid film encountered during operation.

By providing the bearing elements of the invention, adjustable duringoperation, the lubrication conditions can be controlled as desired. Atthe heart of the invention is the fact that, unlike known adjustablebearings, the bearing elements are designed such that the position ofthe bearing surface of an element is determined essentially only by thegeometry of the surface, which is effectively rigid, and by the degreeof adjustment selected. In other words, each adjustable element isimmovable other than by selective adjustment, which can be made duringoperation of the bearing. Such position input to the bearing surface ofan element allows complete control of the lubrication conditions to aprecision previously unattainable. The adjustment is continuouslyvariable, meaning that there is no lower limit to the increment ofadjustment. Unlike foil bearings, the adjustment is by means of aposition adjustment rather than a force adjustment.

For comprehensive control over the conditions within the bearing, theadjustable elements may be adjustable independently of one another.

In a preferred form of the invention, each adjustable bearing element iseffectively hinged about an axis perpendicular to its principaldirection of adjustment. This may be accomplished by way of a truemechanical hinge, but preferably the adjustable bearing element isformed integrally with a part of the bearing relative to which theposition of the element is adjustable, the two being connected by athinned portion which forms the hinge. Further adjustment may beprovided by arranging that the position of the hinge is adjustableduring operation.

The adjustment means for an adjustable bearing element preferablycomprises a support arranged to act on the bearing element, wherein thesupport dimension in the direction of adjustment can be selectivelyaltered.

If appropriate, each support may comprise two or more support elementswhich are adjustable independently of each other to provide a furtherdegree of control over the position of the bearing surface of thebearing element. These support elements may be longitudinally,circumferentially, or radially spaced from one another.

A threaded shaft with a tapered portion, the tapered portion engagingthe bearing element, may be employed to provide the support. In thiscase the threaded shaft is mounted in threaded engagement in the bearingsuch that rotation thereof will advance the tapered portion therebyaltering the support dimension.

Alternatively, the support may be provided by a movable wedge shapedmember engaging the bearing element.

In one form of the invention, the rotating part of the bearing is ashaft journal mounted for operation within a fixed outer casing, theadjustable bearing elements being mounted on and adjustable relative tosaid casing. In another form of the invention the arrangement isreversed, the rotating part of the bearing being an external rotormounted for operation around an inner stator, the adjustable bearingelements being mounted on and adjustable relative to said inner stator.In a further form of the invention, whether the rotating part is a shaftjournal or an outer rotor, the adjustable bearing elements may bemounted on and adjustable relative to said rotating part.

The hydrodynamic bearing preferably comprises at least three adjustablebearing elements.

The bearing elements may be adjustable in a radial direction.Additionally, at least one of the adjustable bearing elements may beadjustable with an axial component of movement to provide axial thrustcapacity. The invention may also be applied to a thrust bearing, inwhich the bearing elements are adjustable in an axial direction tosupport a transverse planar surface of the rotating part of the bearing.

Preferably, the bearing is part of a rotating machine and the positionof the adjustable bearing elements is determined automatically byoperating conditions of the machine. For this purpose, each adjustablebearing element may be provided with a load sensor and/or a displacementsensor.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show how itmay be carried into effect, reference will now be made, by way ofexample only, to the accompanying drawings, in which:

FIG. 1 is a cross section of a bearing in accordance with the invention;

FIGS. 2a and 2b illustrate, in cross section and longitudinal sectionrespectively, a modification to the embodiment of FIG. 1;

FIGS. 3a and 3b depict one possible form of adjustable support for abearing element;

FIGS. 4a and 4b show a bearing in accordance with another embodiment ofthe invention;

FIG. 5 illustrates an adjustable support arrangement for a bearingelement;

FIGS. 6a and 6b show a modified form of bearing element; and

FIG. 7 illustrates the invention applied to a thrust bearing.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 shows a hydrodynamic bearing having a fixed outer casing 10 and arotatable central journal (not shown). The bearing has a plurality (fouras illustrated) of journal support elements in the form of segmentalpads 20 mounted to the outer casing 10 and spaced circumferentially tosurround the journal. Each pad is provided with an inner bearing surface22 of arcuate section. Each pad forms an integral part of a wholebearing component 24, which additionally features a base portion 25firmly fixed to the outer casing 10. The pad and the base portion arejoined by a thinned section 23, such that each pad is effectively hingedto the outer casing 10 by means of the thinned section 23 about an axisparallel to the central axis 11 of the casing 10.

Additionally, an adjustable support 26 is provided between each pad 20and its base portion 25. The precise location of the adjustable support26 on the rear side of the pad (the side distant from the bearingsurface 22) is not of critical importance, so long as it is arrangedsuch that its adjustment will displace the bearing surface 22 bydeflection at the thinned section 23, effectively pivoting the bearingsurface around the axis provided by the thinned section 23. In aprototype tested, the support was arranged in a position on the rearside of the pad corresponding to the maximum lubricant film pressure onthe bearing surface side of the pad--approximately 2/3 of the arcuatelength from the hinge.

The inner bearing surfaces 22 (FIG. 1) are provided on bearing portions21 of the pads and these bearing portions may be provided by casting alayer of white metal to the surface of the pads and machining the innersurface to the required geometry. In operation, therefore, the innerbearing surfaces 22 of the pads 20, along with the cylindrical outersurface of the rotatable journal, define respective lubrication regionsby means of which the journal is supported under fluid pressure forrotation relative to the outer casing 10. Each adjustable support 26 canbe independently controllably adjusted without having to dismantle thebearing and while the journal is rotating, i.e. with the bearing inoperation. By adjusting a support 26, the convergence of the fluid filmbetween the relevant bearing surface 22 and the journal can be adjustedso as to control the lubrication conditions. Since the pads are of veryhigh stiffness and the supports are not deformable, the position of theentire bearing surface is essentially independent of the fluid pressurein the fluid film and, unlike flexible bearing elements such as foils,this gives control over the operation of the bearing to a degree ofprecision hitherto unattainable.

For example, by moving one or more pads (such as twodiametrically-opposed pads adjusted in a common direction), the centralaxis of the rotating journal can be offset from the casing central axis11 to a desired position. Alternatively, or in addition to this controlover the journal eccentricity, the overall stiffness of the bearing canbe increased by adjusting some or all of the pads in an inwardsdirection. The adjustment of the positions of the pads can be controlledmanually, or remotely. Alternatively it can be controlled automaticallyby means of a control system providing signals related to the operatingconditions. For example, the displacements and/or accelerations of theadjustable supports 26 can be determined by the output of amicroprocessor control unit in response to signals representative of thelocation of the central axis of the journal. In this way a desiredposition of the axis of rotation can be maintained under all conditions,such as under the transverse loads to which the shaft of a precisionmachine tool may be subjected, or under conditions of orbital excursionsof the axis of the journal which can occur at critical rotationalspeeds. Alternatively, or in addition, the overall stiffness of thebearing can be controlled by the microprocessor control unit in responseto signals representative of the journal speed, to increase thestiffness at critical ranges (such as low speed or through resonanceranges), or in response to signals representative of the measured load(to avoid the instability that might otherwise occur at very low or zeroload conditions, particularly at high speeds).

The bearing shown in FIG. 1 can be considerably simplified as depictedin FIG. 2a by dispensing with base portion 25 and integrally formingeach pad with the casing 10. In the figure, only one of a plurality ofbearing pads 20 is illustrated. Axial oil supply passage 30 is shown,arranged to allow injection of an appropriate lubricant such as VG32mineral oil into the interior of the casing between the journal and thebearing surface 22 by way of a plurality of supply holes 31 separatedaxially along the leading edge of each converging fluid film wedge. Theoil supply passages are fed from a circumferential common oil channel32. Further openings and channels (not shown), allow the oil, once ithas left the clearance spaces, to return to the common channel. It is tobe noted that the bearing may thus be self-contained, meaning that itcirculates its own lubricant and so requires no external supply. Suchlubricant supply arrangements are known per se and their design does notof itself form a part of the invention.

FIG. 2b shows the same arrangement in longitudinal section, and it is tobe noted that two axially spaced adjustable supports 26 are provided foreach bearing pad 20. The stiffness provided by the use of the twosupports allows pads of reduced thickness to be used and this, alongwith the space saving afforded by the omission of the bearing pad baseportions 25, considerably reduces the overall diameter of the bearingrequired for a journal of given size.

By applying an oscillatory radial motion to the pads 20 via theadjustable supports 26, when oil is present but the journal is notrotating, an oil film may be established in the clearance space and apressure generated therein. The hydrodynamic pressure so generatedmaintains separation between the journal and the bearing surface 22 ofthe pads when the journal is not rotating.

Any number of pads can be used in carrying out the invention, but inorder to afford full adjustment of the journal axis, the minimum numberis three. The adjustable supports 26 and/or the bearing pads 20 may beprovided with load sensors and/or displacement sensors to permitmonitoring of the operation of the bearing or, where appropriate, toprovide input signals for an automatic bearing control system.

In a prototype bearing tested, the following dimensions and materialswere used and found to operate in a satisfactory manner:

    ______________________________________                                        Journal diameter   47.6 mm                                                    Axial length of pad                                                                              25.4 mm                                                    Angle subtended by pad bearing surface                                                           40°                                                 (Arc length 18 mm)                                                            Max. tilt of pad tested                                                                          0.14°                                               (Corresponding to 0.045 mm radial                                             movement at trailing edge)                                                    Oil supply arrangement                                                                           5 jets each of 0.5 mm diameter                             Materials: Body - Mild steel                                                    Bearing surfaces - White metal.                                             ______________________________________                                    

Different forms of adjustable support 26 are possible, provided that thedevice used accomplishes the desired function, namely a non-resilientdisplacement of the bearing surface 22. One such device is a shaft witha tapered portion engaging the pad, the shaft mounted in threadedengagement in an appropriate part (not shown) of the outer casing 10 andaccessible from outside the casing, such that rotation of the shaftadvances (or withdraws) the tapered portion in an axial direction,thereby increasing (or decreasing) the support radial dimension actingon the pad, and hence displacing the pad 20 with respect to the outercasing 10. This device fulfils the desired function in a simple andreliable manner, as it can be arranged to provide a means of adjustmentduring operation (i.e., whilst the bearing journal is rotating) whileensuring that, once adjusted, the pad is rigidly fixed in position.

An alternative adjustable support arrangement employing a cooperatingwedge arrangement is illustrated in FIG. 3a. The bearing pad 20 issupported on a wedge-shaped support element 26 which is itself supportedon a ramp surface 27 provided on the bearing casing 10. The surface 27and the complementary surface of the support element 26 are planar anddisposed at an angle θto the axial direction as shown. The supportelement 26 also has a planar support surface 28 to engage with the rearsurface of the pad 20. A machined recess R forming a longitudinal trackin the rear side of the bearing pad as shown provides a means ofreliably locating the support element 26. FIG. 3b shows a sectional viewof the adjustable support arrangement. Adjustment is by way of athreaded screw 29 in contact with the support element 26. Rotation ofthe screw translates the support element in an axial direction relativeto the casing, thereby moving support surface 28 in a radial directionand hence adjusting the position of bearing surface 22 of the pad.

Another embodiment of the invention is shown in FIG. 4a, in which therotatable part of the bearing is an external rotor (not shown) and thebearing elements are provided on an inner fixed stator 39. The stator isformed from a solid cylindrical piece of metal machined to create thebearing components. FIG. 4b shows the same bearing in cross-sectionthrough plane B--B to illustrate the construction.

The bearing has a plurality (four as illustrated) of bearing elements inthe form of pads 40 machined from the stator piece by removingappropriate material in a roughly U-shaped groove 41 by an appropriatemachining technique, so as to leave pads 40 integral with the stator 39.In a manner similar to that described with reference to the embodimenthaving an inner rotating journal, adjustable supports 46 are provided tocontrol the position of pads 40. These adjustable supports take the formof shafts with tapered support portions and having longitudinal threadedportions engaged in screw fashion in the stator 39 and adjustable fromoutside the stator at ends 47. By rotating a shaft the tapered portionis advanced in an axial direction to alter the diameter of the shaft atthe support position and so radially displace the pad 40. Also shown arelongitudinal shallow grooves 48 machined at the leading edge of each padto define galleries to which oil can be provided by means of oil supplypassages 49 and openings 50. It is to be noted that the entire stator,including all bearing components except for the adjustment shafts, aswell as the lubricant transmission means, are integrally formed. Such adesign has many advantages over a design involving assembly of a numberof different components.

Although only one set of pads is required for realizing this form of theinvention, the stator in FIG. 4a features two sets of pads 40 and 40a,axially separated from one another. Centrally between the two sets ofpads is a further bearing arrangement, namely a hydrostatic bearingportion 60 comprising four circumferentially spaced approximatelyrectangular shallow recesses 61. The hydrostatic bearing portion acts inknown manner with supply means (not shown) of lubricant under pressuresuch that upon starting, and until speeds are attained at which thehydrodynamic fluid film is formed between pads 40/40a and the journal,the journal is adequately supported.

The bearing operates in a similar manner to the embodiment having aninner rotating journal. All eight radially displaceable pads 40/40a ofthe two axially separated sets may be independently adjustable duringoperation, such that the angular alignment in an axial direction, aswell as the eccentricity, of the rotational axis of the rotor may beprecisely controlled.

The cross-sectional shape of the pads is shown in FIG. 4b. Thedisplacement of the pads is afforded by the limited resilientflexibility of the material. The material and geometry, though, is suchthat the shape and position of the bearing surface of a pad isessentially unaffected by changes in the fluid film conditions. Clearly,no material can be completely undeformed by the effects of pressure onits surface. By `essentially unaffected` is meant that a pad issufficiently stiff that any change in position of any part of thebearing surface is substantially less than the thickness of the fluidfilm itself, and this property is common to all embodiments of theinvention. A pad is therefore not conformal to the lubricationconditions, as is the case in bearings such as foil bearings. In thisway, the mechanical properties of a pad allow it to deflect on operationof an adjustment support 46, and thereby to control the lubricationconditions between the bearing surface and the journal, while theselubrication conditions cannot in turn act to deflect the pad itself.This allows the complete and accurate control required.

Additionally the cross-sectional shape of a pad may be designed toequalise stresses in operation across the pad, without affecting theshape of the bearing surface in operation.

In a prototype tested, the following dimensions and materials were used:

    ______________________________________                                        Stator diameter            70 mm                                              Axial length of pad        25.4 mm                                            Angle subtended by pad bearing surface                                                                   72°                                         Max. radial displacement (at trailing edge) of pad tested                                                0.035 mm                                           Materials: Rotor - Mild steel with bearing                                         surface of white metal coating                                                Stator - Mild Steel                                                      ______________________________________                                    

Development work on a bearing according to the invention and on acomputer model has shown that only very small displacements (tiltangles) are required to accommodate a very large range of operatingconditions. The pads may be arranged and mounted to pivot about one edgeby means of a true mechanical hinge. Since no material deflection isrequired of the pads in such a case, they may be of a material oftheoretically infinite stiffness. A disadvantage of such an arrangementis increased complexity, particularly as the pad displacement is thennot resilient and means must be included to positively return the padwhen the radial support dimension is reduced.

The bearing of the present invention has the following advantages overconventional fluid film bearings:

1. The location of the central axis of the rotating member can beadjusted (for example to maintain its position) whilst the bearing is inoperation, without stopping rotation. This may be effected on a steadystate basis (for example, for different applications of a singlemachine) or in response to behaviour through a control system.

2. The static and dynamic characteristics (centre eccentricity andcentre axis inclination of rotating member, stiffness, dampingcoefficients) can all be altered whilst the bearing is in operation,without stopping rotation, and if desired under automatic control.

3. It is possible to compensate for the effects of wear thereby tomaintain properties throughout the operating life of the bearing.

4. The bearing provides a means of avoiding instability (by adjustingthe stiffness and the damping), particularly at low or zero load at highspeed.

5. Bearing seals need not have the tolerances conventionally required,as the position of the rotating element relative to the casing/statorcan be maintained within much closer limits.

6. The control over the fluid film conditions can reduce the lubricanttemperature rise in operation. This and other factors allows operationat higher critical speeds.

It is also possible to provide the adjustable bearing elements on therotating part of the bearing, be it an inner shaft or an external rotor.Clearly some means must be provided to adjust the bearing duringoperation, and this means may either be fully contained within therotating part itself, or may comprise means to pass signals to a controlunit contained within the rotating part to remotely adjust the bearing.For example, commutation means may be incorporated in the bearing, or aradio link may be utilised.

In addition to the tilting form of displacement described in the aboveembodiments, preferred because of the effect such displacement has inincreasing/decreasing the convergence of the wedge-shaped fluid film inthe clearance space, it is possible to adjust the bearing elements inoperation in a linear radial direction, thus not altering the anglebetween the bearing surfaces, but simply changing the separationtherebetween. This may be provided as an alternative to the tilting formof displacement, but it is preferred to combine both forms ofdisplacement by providing radial adjustment of the position of theeffective hinge of the tilting bearing element. In this way, both thethickness and the form of the fluid film can be simultaneously orsequentially varied, to provide yet more control over lubricationconditions within the clearance space.

An alternative bearing element design is illustrated in FIG. 5, whichfeatures two adjustable supports 56a and 56b for each bearing element50. The bearing element is S-shaped in transverse sectional view asshown, such that two effective hinge portions 53a and 53b are providedand adjustment about each hinge portion is realised by operating therespective adjustable support. By adjusting supports 56a and 56b, boththe radial displacement and the tilt angle of bearing surface 52 can befully controlled over a relatively wide range.

In addition to the above-mentioned form of adjustment, it is possible toarrange the adjustable bearing elements, or pads, such that they canmove with an axial component of tilt. In other words, the bearingsurfaces of the pads do not remain parallel to the central axis of thepart of the bearing on which they are supported. In this way there canbe exerted an axial component of fluid film force, to provide a combinedjournal/thrust bearing. The axial thrust capacity can be employed toresist longitudinal forces on a journal, for example. Alternatively,pads adjustable with an axial component of tilt movement can be used toprovide an axial self-centring thrust action for an opposed pair ofbearing elements or of sets of bearing elements, such as those shown inFIG. 4a. Adjustment of the bearing elements is still radial, but tiltingof the pads can take place about an axis transverse to the central axisof the part of the bearing on which they are supported.

A bearing pad 60 for this purpose is illustrated in FIGS. 6a and 6b. Thepad is supported by two axially spaced independently operable adjustablesupports 66. The pad is shaped as shown such that differentialadjustment of the supports 66 will give an axial component of adjustmentby torsional deflection, thus providing a degree of axial bearingcapacity. Once again, though, the bearing pads are designed such thatonce moved into a desired position a pad is fixed there until asubsequent adjustment is selected. The concept may be applied in aconical bearing, where a section of the journal is of frustro-conicalform and the bearing elements are disposed to provide a fluid filmbetween their bearing surfaces and the surface of the conical section ofthe journal.

The invention may also be applied to the hydrodynamic lubrication ofthrust bearings, in which a thrust bearing surface operates on a journalmounted collar, for example. Thrust bearings traditionally suffer fromthe inherent problem that the accuracy to which the perpendicularity ofthe fluid film abutting surface of the collar can be manufactured andmaintained is of similar magnitude to the fluid film thickness. This isdue to manufacturing tolerances and to thermal and elastic distortionsencountered during operation, and gives rise to what is known as`swashplate motion`.

One known form of fluid film thrust bearing incorporates a tilting padarrangement, in which one of the planar sliding surfaces is divided intoa number of segments, each free to take up a position at an angle to theopposing surface in order to conform to the hydrodynamic pressuredistribution, i.e., according to the speed and loading. According to theinvention, these conventional tilting pad bearing elements are replacedby bearing elements adjustable in the axial direction, designed andmounted to be immovable other than by selective adjustment, whichadjustment can be made during operation of the bearing. As with thejournal bearing, position input to the bearing surface of one of thethrust bearing elements allows precise control over the fluid filmconditions and hence over the operation of the bearing. The adjustmentof the position of the bearing surfaces of the respective bearingelements can be arranged to minimise or avoid the so-called swashplatemotion, and thus increase the reliability of the bearing and reduce thepower absorbed. For example, each bearing element can be provided with aload sensor, and the bearing can be arranged with automatic control toafford an equal distribution of the load between all the elements at alltimes. Additionally or alternatively, the device of the invention can beused to provide adjustability of the axial position of the journal andto enhance the bearing axial stiffness and damping.

FIG. 7 illustrates an embodiment of the invention applied to a thrustbearing, in which bearing pads 70 are adjustable with respect to thetransverse planar surfaces 80 of thrust collar 81 which is an integralpart of shaft 82. The figure shows bearing pads 70 arranged in opposedpairs, with the respective pads of each pair on opposite sides of thecollar 81. The pads are adjustable by way of adjustable supports 76. Inthis embodiment, four pairs of bearing pads are arranged equally spacedaround the circumference of the bearing, and a second of the pairs ofpads 70a appears in the illustration. Each pad may be similar in form tothose schematically illustrated in FIGS. 6a and 6b, provided with aplanar bearing surface. A third adjustable support may be provided foreach bearing pad to allow adjustment of the position of the effectivehinge, such that independently adjusting the three supports will allowaccommodation of the swashplate motion described above.

The adjustable supports 26, 46, 56, 66 and 76 may take many forms, andthe two forms already described (axially movable tapered shaft andcooperating wedge arrangement) are only two possible forms. Otherpossibilities which enable a non-resilient position input include a cam,rotatable to alter the radial or axial support dimension, apiezo-electric element, arranged to undeformably alter its radial oraxial dimension in response to an electric charge, and amagneto-restrictive device.

The present invention has many different applications as will be evidentto an engineer. For example, there is an ever-increasing demand for moreaccuracy in the production of components, and the invention will allowprecise control in the operation of machine tools used in suchproduction. In particular, the bearing of the invention can be employedin situations where hitherto roller element bearings have been used dueto the requirement of very accurate shaft location capability (such asin helicopter transmissions). A significant advantage arises from thefact that a fluid film bearing involves a far greater minimum oilthickness than that of a rolling element bearing, thus reducing thevulnerability of the bearing components to foreign object damage.Studies by the inventors have suggested that the minimum film thicknessis 15 times that involved in a rolling element bearing.

Other non-limiting applications of the invention include processingequipment such as printing rollers and handling conveyors, turbines,compressors, pumps, aeronautical and marine engines, gearboxes and othercomponents. The invention is also particularly applicable to machineswhere large speed ranges are involved.

A gas bearing version may find useful application in environments whereavoidance of contamination by lubricant is of great importance, such asin machinery used in the food industry, in computer drive applications,and in the field of gas turbines.

We claim:
 1. A hydrodynamic bearing having means for supporting a rotating part of the bearing in operation on a film of lubrication fluid, said means comprising a plurality of circumferentially spaced bearing elements having respective bearing surfaces defining areas of support for the rotating part, at least some of the bearing elements being adjustable during operation to vary lubrication conditions in the fluid film, wherein the adjustable bearing elements are of sufficiently high stiffness that the position of the entire bearing surface of a bearing element is essentially independent of the pressure in said fluid film encountered during operation.
 2. A hydrodynamic bearing according to claim 1, wherein the adjustable elements are independently adjustable.
 3. A hydrodynamic bearing according to claim 1 or claim 2, wherein each adjustable bearing element is provided with a hinge structure so as to be effectively hinged about an axis perpendicular to a principal direction of adjustment thereof.
 4. A hydrodynamic bearing according to claim 3, wherein the adjustable bearing element is formed integrally with a part of the bearing relative to which the position of the element is adjustable, the two being connected by a thinned portion which forms the hinge structure.
 5. A hydrodynamic bearing according to claim 3 in which the position of the hinge structure is adjustable during operation.
 6. A hydrodynamic bearing according to claim 1, wherein each adjustable bearing element is adjustable by way of a support arranged to act on the bearing element, wherein the support dimension in the direction of adjustment can be selectively altered.
 7. A hydrodynamic bearing according to claim 6, wherein each support comprises two or more support elements which are adjustable independently of each other.
 8. A hydrodynamic bearing according to claim 6 or claim 7, comprising a threaded shaft with a tapered portion, the tapered portion engaging the bearing element to provide said support, and the threaded shaft being mounted in threaded engagement in the bearing such that rotation thereof will advance the tapered portion thereby altering the support dimension.
 9. A hydrodynamic bearing according to claim 6 or claim 7, comprising a movable wedge shaped member engaging the bearing element to provide said support.
 10. A hydrodynamic bearing according to claim 1, wherein the rotating part is a shaft journal mounted for operation within a fixed outer casing, the adjustable bearing elements being mounted on and adjustable relative to said casing.
 11. A hydrodynamic bearing according to claim 1, wherein the rotating part is an external rotor mounted for operation around an inner stator, the adjustable bearing elements being mounted on and adjustable relative to said inner stator.
 12. A hydrodynamic bearing according to claim 1, wherein the adjustable bearing elements are mounted on and adjustable relative to the rotating part.
 13. A hydrodynamic bearing according to claim 1 comprising at least three adjustable bearing elements.
 14. A hydrodynamic bearing according to claim 1, wherein the adjustable bearing elements are adjustable in a radial direction.
 15. A hydrodynamic bearing according to claim 14, wherein at least one of the adjustable bearing elements is adjustable with an axial component of movement to provide axial thrust capacity.
 16. A hydrodynamic bearing according to any of claim 1, wherein the bearing is a thrust bearing, the bearing elements being adjustable in an axial direction to support a transverse planar surface of the rotating part of the bearing.
 17. A rotating machine incorporating at least one hydrodynamic bearing, each said hydrodynamic bearing having means for supporting a rotating part of the bearing in operation on a film of lubrication fluid, said means comprising a plurality of circumferentially spaced bearing elements having respective bearing surfaces defining areas of support for the rotating part, at least one of the bearing elements being adjustable during operation to vary lubrication conditions in the fluid film, wherein the adjustable bearing elements are of sufficiently high stiffness that the position of the entire bearing surface of a bearing element is essentially independent of the pressure in said fluid film encountered during operation. 